1. Field of the Invention
This invention pertains generally to fabrication of metal hydroxide, phosphate, and oxide semiconductor, photovoltaic and optoelectronic materials, and more particularly to a method for the fabrication of nanostructured semiconducting, photoconductive, photovoltaic, optoelectronic and electrical battery thin films and materials at low temperature, with no molecular template and no organic contaminants, as well as materials fabricated therefrom.
2. Description of Related Art
There are a variety of methods used to deposit layers of material on a substrate, many of which depend on high vacuum and high-energy deposition methods. The three methods listed below are commonly used in research, development, and manufacturing of semiconducting and photovoltaic thin films.
Metal organic chemical vapor deposition (MOCVD) is used to deposit a variety of thin films on solid substrates and enjoys wide application due to the widespread usefulness of oxide superconductors, ferroelectrics, and dielectric materials. This method is the dominant growth technique behind novel device fabrication and is the popular choice of manufacturers in high volume production of epitaxial wafers (the most common form of “computer chips”) and devices. The method utilizes a high vacuum chamber and a heated substrate. Simple organometallic compounds are vaporized and passed into the ultra-high vacuum (UHV) chamber and decompose upon contact with the heated substrate leaving behind the metal atom on the surface. The carbon byproduct of the reaction is transported out of the chamber by an inert carrier gas. The parameters for deposition employ growth temperatures from 950-1025° C. and deposition rates of 1-4 μm per hour. Using MOCVD, high purity thin films of a variety of compositions can be deposited on a surface. The kinetic and thermodynamic parameters of the film deposition process are governed by the crucial interdependence of precursor composition, deposition temperature, partial pressure and flow rate. The search for effective precursors and process conditions is active and the balance between precursor volatility and thermal stability remains a particularly difficult challenge.
Molecular beam epitaxy (MBE) is a UHV (10−10 to 10−14 torr) deposition process that introduces reactants using a molecular beam. The beam is created by heating an elemental source in a UHV chamber; it then effuses through a small orifice towards the substrate and subsequently is deposited.
Using several sources compound materials can be deposited in as little as one atomic layer. This precise control of film growth results in very accurate material composition with small amounts of defects. Growth occurs one monolayer at a time giving high purity layers but at the cost of very slow deposition rates of usually 0.1-0.5 μm/hr. Therefore MBE is very costly, and only is commercially useful for thin layers typically no more than a few nanometers in thickness.
Liquid phase epitaxy (LPE) is a process in which a substrate is brought into contact with a molten saturated solution of the film material at a temperature high enough to melt a solid source. The substrate is then cooled to initiate crystallization of the semiconductor and its growth as a film on the surface of the substrate. The control of stoichiometry is good in this method and the level of defect formation is low. The solubility of the film constituents is a major limiting factor, however, and application of this method is therefore quite limited. An additional drawback to this method is the fact that morphology of the resulting surface is difficult to control and the surface is often heterogeneous with respect to composition and morphology. Large step edges on the surface called macrosteps are formed during LPE which impede the resolution of structures on the surface. These steps also are a source for compositional inhomogeneities due to build-up of impurities at the step edge. LPE is a high-energy process that relies on high purity solid melts in contact with a cold substrate. The temperature control needs to be precise to prevent internal stresses and cracking due to temperature gradients. The thermal expansion mismatch between a substrate and thin film needs to be engineered and therefore limitations in material combinations are a major problem that plagues this method.
Alternative routes to high purity semiconductor materials to replace these techniques are being explored in response to demands for more flexible and lower energy synthesis strategies. Techniques that mimic biomineralization have received much attention because of the inherently benign conditions of biological syntheses. In addition, these biomineralization processes often produce highly ordered structures on the nanoscopic as well as macroscopic scale.
One such biomineralization process has been studied intensively. Molecular cloning, sequence analyses, and mechanistic studies of the biological synthesis of silica structures in a marine sponge has led to the discovery that this process is mediated by a family of catalytically active, structure-directing enzymes called silicateins.
Purified silicatein fibers are able to catalyze and structurally direct the hydrolysis and polycondensation of silicon alkoxides at low temperature and neutral pH. Silicatein also was used as a catalyst and template for the hydrolysis and subsequent polycondensation of water stable molecular complexes of titanium and gallium to form nanocrystalline TiO2 and Ga2O3, respectively. However, these nanoparticles remain in intimate contact with the macroscopic (2 μm×1 mm) protein filaments that catalyzed and templated their synthesis; they are therefore largely unsuitable for device applications that require high purity material.
Perovskite materials are of interest because they have a wide range of useful applications, for example, in ferroelectric random access memory (FeRAM), piezoelectric transducers, solid-oxide fuel cells, high-temperature superconductors, thermoelectrics, ferromagnets, capacitors, pyroelectric detectors, and colossal magnetoresistors. Perovskites are traditionally prepared by high-temperature solid-state reactions; specifically, BaTiO3 is prepared by the reaction of TiO2 and BaCO3 at temperatures above 1100° C., which yields a wide range of grain sizes (0.5-3 μm) and provides very little control over the shape of the particles. Consequently, lower-temperature solution-based synthetic routes (for example, sol-gel and hydrothermal methods) are being explored to better control the nanostructure of the BaTiO3 product. The synthesis of BaTiO3 nanoparticles of 6-12 nm by a sol-gel reaction at 100-140° C., in the presence of oleic acid as a stabilizing agent, has been reported. Other researchers have described a nonhydrolytic synthesis of 6 nm BaTiO3 nanoparticles at 200-220° C. The hydrothermal synthesis of well-defined 17 nm BaTiO3 nanoparticles at 180° C. under highly alkaline conditions has also been reported. The low-temperature synthesis of perovskite nanocrystals is inherently difficult; an accurate control over stoichiometry, a close matching of the reaction rates of the precursors, and the identification of special conditions for crystallization (that is, temperature, pressure, and pH) are all critically important. While several examples of successful controlled syntheses of BaTiO3 nanostructures now exist, they rely on elevated reaction temperatures (>140° C.) and/or strongly alkaline conditions for crystallization.